Exhaust gas purification system of internal combustion engine

ABSTRACT

An exhaust gas purification system of an internal combustion engine supplies hydrocarbon by performing a post injection and the like in order to combust particulate matters accumulated in a diesel particulate filter (a DPF) having an oxidation catalyst. An electronic control unit (an ECU) senses temperature of exhaust gas upstream of the DPF with an exhaust gas temperature sensor and determines an upper limit value of permissible quantity of the hydrocarbon supplied to the DPF based on the sensed temperature. The ECU controls post injection quantity so that the quantity of the actually supplied hydrocarbon does not exceed the upper limit value. Thus, hydrocarbon poisoning can be precluded, since the quantity of the actually supplied hydrocarbon does not exceed the upper limit value.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on and incorporates herein by referenceJapanese Patent Application No. 2003-5595 filed on Jan. 14, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an exhaust gas purificationsystem of an internal combustion engine having an exhaust gasafter-treatment device such as a particulate filter, which has acatalyst, in an exhaust passage. Specifically, the present inventionrelates to an exhaust gas purification system having means for avoidingcatalyst poisoning due to hydrocarbon supplied to an exhaust gasafter-treatment device.

[0004] 2. Description of Related Art

[0005] Recently, an exhaust gas purification system for treating gasdischarged from an internal combustion engine with a catalyst or afilter in order to inhibit discharge of harmful components has beenemphasized as environment-protecting measures. As an example of such anexhaust gas purification system, there is an exhaust gas purificationsystem for collecting particulate matters discharged from a dieselengine with a diesel particulate filter (a DPF) disposed in an exhaustpipe. The exhaust gas purification system combusts and eliminates thecollected particulate matters regularly. Thus, the DPF is regeneratedand can be used continuously.

[0006] The regeneration of the DPF is performed when a quantity of theparticulate matter accumulated on the surface of the DPF (a PMaccumulation quantity) becomes higher than a predetermined quantity. ThePM accumulation quantity is calculated based on a pressure differenceacross the DPF. In order to regenerate the DPF, unburned hydrocarbon(HC) is supplied to the DPF by performing a post injection and the like.The supplied hydrocarbon is combusted with an oxidization catalyst,which is supported on the surface of the DPF in advance. Heat generatedthrough the combustion of the unburned hydrocarbon can increase thetemperature of the DPF above a certain temperature (for instance, 600°C.), above which the particulate matters can be combusted.

[0007] However, if the temperature of the DPF is low at that time,velocity of the catalytic reaction is reduced. If a large amount of thehydrocarbon is supplied in that state, the supplied hydrocarbon willadhere to a surface of the catalyst. As a result, diffusion of theexhaust gas including the harmful components to the neighborhood of thecatalyst is inhibited, so the catalytic reaction is hindered. Thus, aproblem of catalyst poisoning due to the hydrocarbon (hydrocarbonpoisoning) is caused. The hydrocarbon poisoning is caused by physicaladhesion of the hydrocarbon to the catalyst. Therefore, catalyticactivity can be restored by holding the DPF at high temperature and bypromoting detachment of the hydrocarbon adhering to the catalyst. Atechnology employing the above principle and relating to a regeneratingoperation of the catalyst poisoned with the hydrocarbon is disclosed inJapanese Patent Unexamined Publication No. H11-257125 (a patent document1), for instance.

[0008] A system for purifying nitrogen oxides (NOx) with the catalyst isdisclosed in the patent document 1. This system includes determiningmeans for calculating a quantity of the hydrocarbon adhering to thecatalyst and for determining the catalyst poisoning based on whether theadhering quantity of the hydrocarbon is greater than a predeterminedquantity or not. Thus, the system performs an operation for eliminatingthe catalyst poisoning when it is determined that the catalyst ispoisoned. The operation for eliminating the catalyst poisoning isperformed by closing an intake throttle valve and by opening an exhaustgas recirculation valve (an EGR valve) of the exhaust gas. Thus, thetemperature decrease of the catalyst is prevented. Moreover, thedetachment and the oxidation of the hydrocarbon are promoted and thetemperature of the catalyst is increased with the oxidative reactionheat. In the system, the hydrocarbon is added upstream of the catalystin order to reduce and purify the nitrogen oxides.

[0009] More specifically, the method disclosed in the patent document 1permits the generation of the catalyst poisoning due to the hydrocarbonand takes measures only after the hydrocarbon poisoning is generated.However, if the catalyst is once poisoned with the hydrocarbon, it takeslong time to eliminate the poisoning. Moreover, the purification of theharmful components cannot be performed until the hydrocarbon poisoningis eliminated. Specifically, if an inlet end surface of the DPF isexposed to the high-temperature exhaust gas after the inlet end surfaceis poisoned with the hydrocarbon, the adhering hydrocarbon will becarbonized. There is a possibility that the carbonized hydrocarbon mayblock the inlet end surface of the DPF in the worst case. In order toeliminate the carbonized hydrocarbon, the carbonized hydrocarbon must beburned by holding the blocked portion at very high temperature (600° C.or higher, for instance) for a long time. However, it is difficult tomaintain the high temperature for a long time in a normal operatingstate occupying a major part of actual travel, where the temperature ofthe exhaust gas is low (300° C. or lower, for instance). As a result,fuel consumption will be increased largely.

[0010] Therefore, the hydrocarbon poisoning itself should be avoidedinstead of taking the measures after the hydrocarbon poisoning iscaused. Specifically, the DPF requires the supply of the large amount ofthe hydrocarbon through the post injection and the like in order tocombust the accumulated particulate matters. Therefore, the DPF is proneto be poisoned with the hydrocarbon. If the hydrocarbon poisoning iscaused, the temperature of the DPF cannot be increased quickly, so theparticulate matters cannot be combusted suitably. Therefore, avoidanceof the hydrocarbon poisoning is important.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the present invention to provide anexhaust gas purification system of an internal combustion engine capableof avoiding poisoning of a catalyst, which is disposed in an exhaust gasafter-treatment device, due to hydrocarbon. Thus, an operation forrecovering catalytic activity can be omitted, so deterioration ofcapability of combusting particulate matters or capability of purifyingharmful components due to the catalytic activity recovering operationcan be prevented. Meanwhile, carbonization of the adhering hydrocarboncan be prevented. Thus, performance of the catalyst can be maintainedfor a long time and a highly reliable device can be provided.

[0012] According to an aspect of the present invention, an exhaust gaspurification system of an internal combustion engine includes an exhaustgas after-treatment device, temperature sensing means, hydrocarbonsupplying means, and hydrocarbon supply quantity controlling means. Theexhaust gas after-treatment device is disposed in an exhaust passage ofthe internal combustion engine and supports a catalyst on its surface.The temperature sensing means estimates the temperature of the exhaustgas after-treatment device. The hydrocarbon supplying means supplieshydrocarbon to the exhaust gas after-treatment device. The hydrocarbonsupply quantity controlling means determines an upper limit value of apermissible quantity of the hydrocarbon supplied to the exhaust gasafter-treatment device in accordance with the temperature of the exhaustgas after-treatment device, which is estimated by the temperaturesensing means. The hydrocarbon supply quantity controlling meanscontrols the hydrocarbon supplying means so that the quantity of thehydrocarbon supplied to the exhaust gas after-treatment device does notexceed the upper limit value.

[0013] The temperature of the exhaust gas after-treatment device greatlyaffects the poisoning due to the hydrocarbon (the hydrocarbonpoisoning). Therefore, the upper limit value of the permissible quantityof the supplied hydrocarbon is determined in accordance with thetemperature of the exhaust gas after-treatment device. Meanwhile, thehydrocarbon supplying means is controlled so that the quantity of thesupplied hydrocarbon does not exceed the upper limit value. Thus, thehydrocarbon poisoning itself can be prevented, and an operation forrecovering catalytic activity is unnecessary. Therefore, problems ofdecrease in purifying performance during the catalytic abilityrecovering operation or carbonization of adhering hydrocarbon can beprevented. Thus, a highly reliable device with high catalyticperformance can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Features and advantages of an embodiment will be appreciated, aswell as methods of operation and the function of the related parts, froma study of the following detailed description, the appended claims, andthe drawings, all of which form a part of this application. In thedrawings:

[0015]FIG. 1 is a schematic diagram showing an exhaust gas purificationsystem of an internal combustion engine according to an embodiment ofthe present invention;

[0016]FIG. 2 is a graph showing an area where hydrocarbon poisoning iscaused, based on coordinate axes of DPF inlet gas temperature and DPFinlet gas hydrocarbon quantity;

[0017]FIG. 3 is a graph showing a quantity of hydrocarbon generatedthrough combustion in the engine, based on coordinate axes of enginerotation speed and output torque;

[0018]FIG. 4 is a graph showing post injection quantity based oncoordinate axes of the engine rotation speed and the output torque;

[0019]FIG. 5 is a graph showing a relationship between the postinjection quantity and combusted part of the post injection quantitybased on the output torque;

[0020]FIG. 6 is a flowchart showing control performed by an electroniccontrol unit of the exhaust gas purification system according to thepresent embodiment; and

[0021]FIG. 7 is a time chart showing an effect of the exhaust gaspurification system according to the present embodiment.

DETAILED DESCRIPTION OF THE REFERRED EMBODIMENT

[0022] Referring to FIG. 1, an exhaust gas purification system of adiesel engine 1 according to the present embodiment is illustrated. Adiesel particulate filter (a DPF) 3 as an exhaust gas after-treatmentdevice is disposed between an upstream exhaust pipe 2 a and a downstreamexhaust pipe 2 b as exhaust passages of the engine 1. The DPF 3 supportsan oxidization catalyst on its surface. For instance, the DPF 3 isformed of heat-resistant ceramics such as cordierite in the shape of ahoneycomb having a multiplicity of cells as gas passages. An inlet or anoutlet of each cell of the DPF 3 is blocked alternately. The oxidationcatalyst such as platinum is applied on the surfaces of cell walls ofthe DPF 3. Exhaust gas discharged from the engine 1 flows downstreamwhile passing through the porous partition walls of the DPF 3.Meanwhile, particulate matters included in the exhaust gas are collectedby the partition walls and are gradually accumulated in the DPF 3.

[0023] An exhaust gas temperature sensor 41 is disposed in the upstreamexhaust pipe 2 a upstream of the DPF 3. Another exhaust gas temperaturesensor 42 is disposed in the downstream exhaust pipe 2 b downstream ofthe DPF 3. The exhaust gas temperature sensors 41, 42 are connected toan electronic control unit (an ECU) 6. The exhaust gas temperaturesensor 41 senses temperature of the exhaust gas at an inlet of the DPF 3and outputs the temperature to the ECU 6. The exhaust gas temperaturesensor 42 senses the temperature of the exhaust gas at an outlet of theDPF 3 and outputs the temperature to the ECU 6. The upstream exhaust gastemperature sensor 41 is used as temperature sensing means forestimating the temperature of the DPF 3 in hydrocarbon supply quantitycontrol (explained after). An airflow meter (an intake quantity sensor)43 is disposed in an intake pipe 11 of the engine 1. The airflow meter43 senses air intake quantity and outputs the intake quantity to the ECU6. The intake pipe 11 is connected with the upstream exhaust pipe 2 aupstream of the DPF 3 through an exhaust gas recirculation passage (anEGR passage) 71 having an exhaust gas recirculation valve (an EGR valve)7. The ECU 6 controls the drive of the EGR valve 7.

[0024] A pressure difference sensor 5 is connected to the upstreamexhaust pipe 2 a and the downstream exhaust pipe 2 b for measuring aquantity of the particulate matters collected and accumulated in the DPF3 (a particulate matter accumulation quantity, hereafter) by sensing apressure difference across the DPF 3. An end of the pressure differencesensor 5 is connected with the upstream exhaust pipe 2 a through apressure introduction pipe 51. The other end of the pressure differencesensor 5 is connected with the downstream exhaust pipe 2 b throughanother pressure introduction pipe 52. The pressure difference sensor 5outputs a signal corresponding to the pressure difference across the DPF3 to the ECU 6. The ECU 6 calculates the particulate matter accumulationquantity based on the pressure difference across the DPF 3. The ECU 6performs regeneration control of the DPF 3 when the particulate matteraccumulation quantity exceeds a predetermined quantity.

[0025] Moreover, the ECU 6 is connected with various sensors such as anaccelerator position sensor 61 or a rotation speed sensor 62. The ECU 6detects an operating condition based on detection signals outputted fromthe various sensors and calculates optimum fuel injection quantity,injection timing, injection pressure and the like in accordance with theoperating condition. Thus, the ECU 6 controls fuel injection into theengine 1. The ECU 6 controls a quantity of the exhaust gas recirculatedto the intake air (an EGR quantity) by regulating an opening degree ofthe EGR valve 7.

[0026] The ECU 6 includes hydrocarbon supplying means for supplying thehydrocarbon to the DPF 3 in order to combust the accumulated particulatematters in the regeneration of the DPF 3. The supplied hydrocarbon iscombusted by the oxidation catalyst supported on the surface of the sellwalls of the DPF 3. The heat generated through the combustion of thehydrocarbon increases the temperature of the DPF 3 above a certaintemperature, at which the particulate matters can be combusted. In orderto supply the hydrocarbon, the hydrocarbon supplying means performs anoperation such as a post injection, retardation of fuel injection timingand restriction of the intake air, or a combination of them.

[0027] However, if the temperature of the DPF 3 is low at that time,there is a possibility that the hydrocarbon poisoning may be caused. Ifa large amount of the hydrocarbon is supplied to the catalyst when thevelocity of the oxidation reaction by the oxidation catalyst is lowbecause of the low temperature of the catalyst, the supplied hydrocarbonwill adhere to the surface of the catalyst. As a result, the catalyticreaction is hindered because diffusion of the gas including the harmfulcomponents to the neighborhood of the catalyst (the platinum and thelike) is inhibited. This phenomenon is the hydrocarbon poisoning. Thehydrocarbon poisoning is caused by physical adhesion of the hydrocarbonto the catalyst. Therefore, the hydrocarbon poisoning can be eliminatedreversibly by decreasing the hydrocarbon supply quantity and by holdingthe catalyst at high temperature.

[0028] Therefore, in the present embodiment, the ECU 6 includeshydrocarbon supply quantity controlling means for determining an upperlimit value of the permissible quantity of the hydrocarbon supplied tothe DPF 3 in accordance with the temperature of the DPF 3 and forcontrolling the hydrocarbon supplying means so that the quantity of theactually-supplied hydrocarbon does not exceed the upper limit value. Agraph shown in FIG. 2 is based on experimentation performed by theinventors. The graph in FIG. 2 shows an area “A” where the hydrocarbonpoisoning is not caused and another area “B” where the hydrocarbonpoisoning is caused, based on axes of the exhaust gas temperatureupstream of the DPF 3, or DPF inlet gas temperature Tin (° C.), and thequantity of the hydrocarbon supplied to the DPF 3, or an inlet gashydrocarbon quantity HCin (g/min). In FIG. 2, cross marks representexperimental results, in which the hydrocarbon poisoning is caused.Round marks represent the experimental results, in which the hydrocarbonpoisoning is not caused. In FIG. 2, it is determined that thehydrocarbon poisoning is caused if the change in the DPF temperatureTDPF (or an increase in the DPF temperature TDPF per unit time) afterthe hydrocarbon is supplied is less than a predetermined value.Otherwise, it is determined that the hydrocarbon poisoning is notcaused. It is because the catalyst temperature will increase quickly ifthe hydrocarbon poisoning is not caused, and the increase in thecatalyst temperature will be reduced if the hydrocarbon poisoning iscaused.

[0029] As shown in FIG. 2, whether the hydrocarbon poisoning is causedor not greatly depends on the temperature Tin of the exhaust gas, whichincludes the hydrocarbon supplied to the DPF 3. If the temperature Tinof the exhaust gas upstream of the DPF 3 is equal to or lower than 250°C., the possibility of the hydrocarbon poisoning is very high. If thetemperature of the exhaust gas upstream of the DPF 3 is higher than 250°C., the possibility of the hydrocarbon poisoning is reduced as thetemperature Tin of the exhaust gas upstream of the DPF 3 increases. Inthis case, the upper limit value HCup of the permissible quantity of thehydrocarbon supplied to the DPF 3 is increased as the temperature Tin ofthe exhaust gas upstream of the DPF 3 increases. The upper limit valueHCup of the hydrocarbon for preventing the hydrocarbon poisoning isdetermined based on the graph in FIG. 2, assuming that the DPFtemperature TDPF coincides with the DPF inlet gas temperature Tin sensedby the exhaust gas temperature sensor 41. The ECU 6 includes hydrocarbonquantity sensing means for sensing the quantity of the hydrocarbon,which is included in the exhaust gas and is supplied to the DPF 3. Then,the ECU 6 determines whether the hydrocarbon poisoning will be caused ornot by comparing the sensed hydrocarbon quantity with the upper limitvalue. The hydrocarbon quantity sensing means calculates the quantityHCact of the hydrocarbon actually supplied to the DPF 3 from acombustion-originated quantity HCcom of the hydrocarbon generatedthrough the combustion in the engine 1 and a supply-originated quantityHCsup of the hydrocarbon supplied by the hydrocarbon supplying means,based on a following formula (1).

HCact=HCcom+HCsup,  (1)

[0030] More specifically, the quantity HCact of the hydrocarbon actuallysupplied to the DPF 3, which causes the hydrocarbon poisoning, is thesummation of the combustion-originated quantity HCcom of the hydrocarbongenerated through the combustion in the engine 1 and thesupply-originated quantity HCsup of the hydrocarbon supplied by thehydrocarbon supplying means through the post injection and the like.

[0031] More specifically, the combustion-originated quantity HCcom ofthe hydrocarbon is calculated based on the operating condition of theengine such as engine rotation speed NE and the output torque.

[0032] For instance, a graph in FIG. 3 shows the combustion-originatedquantity HCcom of the hydrocarbon generated through the combustion inthe engine 1, based on axes of the engine rotation speed NE and theoutput torque. The combustion-originated quantity HCcom increases alongan arrow mark in FIG. 3. A solid line “MAX” in FIG. 3 represents themaximum value of the output torque. The relationship between theoperating condition and the combustion-originated quantity HCcom isstored in advance, so the combustion-originated quantity HCcom of thehydrocarbon can be calculated easily from the engine rotation speed NEand the output torque.

[0033] The supply-originated quantity HCsup of the hydrocarbon suppliedby the hydrocarbon supplying means can be calculated based on anoperation degree of the hydrocarbon supplying means (for instance, thepost injection quantity) and the engine condition (for instance, theoutput torque).

[0034] A graph in FIG. 4 shows the post injection quantity Qpost undervarious engine operating conditions based on axes of the engine rotationspeed NE and the output torque. The post injection quantity Qpostincreases along an arrow mark in FIG. 4. The post injection quantityQpost is basic post injection quantity adjusted for each engineoperating condition in advance. Part of the fuel injected in the postinjection is combusted in the cylinder. Therefore, the post injectionquantity Qpost does not necessarily coincide with the actual quantity ofthe hydrocarbon supplied through the post injection. Therefore, in thepresent embodiment, the supply-originated quantity HCsup of thehydrocarbon supplied by the hydrocarbon supplying means is calculated,while considering the quantity Qcom of the hydrocarbon combusted in thecylinder.

[0035] The quantity HCsup of the hydrocarbon actually supplied to theDPF 3 through the post injection varies in accordance with the engineoutput torque and the like. It is because the quantity Qcom of the fuelcombusted in the cylinder, which is part of the post injection quantityQpost, changes in accordance with the temperature in the cylinder andthe post injection timing. Therefore, the quantity Qcom of the fuelcombusted in the cylinder in the post injection quantity Qpost iscalculated based on a graph in FIG. 5 showing the relationship betweenthe post injection quantity Qpost and the quantity Qcom of the fuelcombusted in the cylinder in the post injection quantity Qpost. In FIG.5, a solid line “TL” represents the combusted quantity Qcom at the timewhen the output torque is large. A solid line “TM” represents thecombusted quantity Qcom at the time when the output torque is medium. Asolid line “TS” represents the combusted quantity Qcom at the time whenthe output torque is small. The supply-originated quantity HCsup of thehydrocarbon supplied by the hydrocarbon supplying means can becalculated from the basic post injection quantity Qpost and the quantityQcom of the fuel combusted in the cylinder in the post injectionquantity Qpost, based on a following formula (2). In the formula (2), C1represents a constant value for converting the quantity of the injectedfuel into the quantity of the hydrocarbon.

HCsup=(Qpost−Qcom)×C 1,  (2)

[0036] The ECU 6 compares the quantity of the supplied hydrocarbon,which is calculated by the hydrocarbon quantity sensing means, with theupper limit value HCup of the hydrocarbon quantity for preventing thehydrocarbon poisoning. If the quantity of the supplied hydrocarbonexceeds the upper limit value HCup, the ECU 6 determines that thehydrocarbon poisoning can be caused and the quantity of the hydrocarbonsupplied by the hydrocarbon supplying means through the post injectionand the like is decreased. Thus, the generation of the hydrocarbonpoisoning can be prevented by controlling the quantity of the suppliedhydrocarbon below the upper limit value HCup.

[0037] The basic post injection quantity Qpost shown in FIG. 4 iscalculated through bench testing of the engine 1 in a state where thetemperature of the DPF 3 is stabilized sufficiently. Generally, in orderto increase the. temperature of the DPF 3 quickly, the basic postinjection quantity Qpost is determined so that the actual post injectionquantity becomes as great as possible. Therefore, there is a possibilitythat the temperature of the DPF 3 at the time when the system isactually mounted on a vehicle and is operated does not coincide with thetemperature of the DPF 3 at the time when the basic post injectionquantity Qpost is calculated in the bench testing of the engine 1 evenif the operating conditions of the engine 1 are the same. In particular,such a situation will occur in a period immediately after the enginestart or in the early stage of the acceleration, for instance. In orderto avoid such a situation, the system of the present embodiment correctsthe supplying quantity of the hydrocarbon and supplies the appropriatequantity of the hydrocarbon. Thus, the hydrocarbon poisoning can beprevented.

[0038] Next, hydrocarbon supply quantity control performed by the ECU 6will be explained based on a flowchart shown in FIG. 6.

[0039] First, in Step S101, it is determined whether the hydrocarbonsupplying means is supplying the hydrocarbon to the DPF 3 or not. In thepresent embodiment, it is determined whether the condition forperforming the post injection is established or not in Step S101. If theresult of the determination in Step S101 is “YES”, the processingproceeds to Step S102. In Step S102, the engine rotation speed NE andthe accelerator position ACCP are inputted from the rotation speedsensor 62 and the accelerator position sensor 61. Meanwhile, thetemperature Tin of the exhaust gas upstream of the DPF 3 is inputtedfrom the exhaust gas temperature sensor 41. If the result of thedetermination in Step S101 is “NO”, the processing is ended withoutperforming the post injection.

[0040] The post injection is performed to regenerate the DPF 3 bycombusting the particulate matters when the particulate matteraccumulation quantity exceeds the predetermined quantity. Morespecifically, a small amount of the fuel is additionally injected afterthe main fuel injection performed for operating the engine, or during anexpansion stroke after the top dead center. Thus, the unburnedhydrocarbon is generated and the hydrocarbon is supplied to the DPF 3.The hydrocarbon supplied to the DPF 3 is combusted by the catalystsupported on the surface of the DPF 3. The DPF 3 is heated to hightemperature (above 500° C., for instance) by the combustion heat, so theparticulate matters on the surface of the DPF 3 are combusted. Thesimilar effect can be exerted by retarding the fuel injection timing orby increasing the EGR quantity, in addition to the post injection.

[0041] Whether the particulate matter accumulation quantity has reachedthe predetermined quantity or not is determined by comparing theparticulate matter accumulation quantity with the predeterminedquantity. The particulate matter accumulation quantity is calculatedbased on the pressure difference across the DPF 3 sensed by the pressuredifference sensor 5. The particulate matter accumulation quantity can becalculated based on the pressure difference across the DPF 3 because thepressure difference, which is generated when a predetermined quantity ofthe exhaust gas passes through the DPF 3, is correlated with theparticulate matter accumulation quantity. The correlation is obtained inadvance through experimentation and the like. The quantity of theexhaust gas is calculated from the air intake quantity sensed by theairflow meter 43, the temperature of the upstream side and thedownstream side of the DPF 3 sensed by the exhaust gas temperaturesensors 41, 42, the pressure difference sensed by the pressuredifference sensor 5 and the like.

[0042] Then, in Step S103, the combustion-originated quantity HCcom ofthe hydrocarbon discharged from the engine 1 is calculated from theengine rotation speed NE and the engine output torque calculated fromthe accelerator position ACCP, based on the relationship shown in FIG.3. Then, in Step S104, the basic post injection quantity Qpost iscalculated from the engine rotation speed NE and the engine outputtorque, based on FIG. 4.

[0043] Then, in Step S105, the quantity Qcom of the fuel combusted inthe cylinder in the post injection quantity Qpost is calculated form thebasic post injection quantity Qpost calculated in Step S104 and theengine output torque, based on FIG. 5. As shown in FIG. 5, as the outputtorque increases, the quantity Qcom of the fuel combusted in thecylinder increases and the temperature in the cylinder increases.Therefore, even if the post injection quantity Qpost is constant, thequantity Qcom of the fuel combusted in the cylinder increases as theoutput torque increases.

[0044] Then, in Step S106, the quantity HCact of the hydrocarbonactually supplied to the DFP 3 is calculated. First, supply-originatedquantity HCsup of the hydrocarbon originating from the post injection iscalculated from the basic post injection quantity Qpost and the quantityQcom of the fuel combusted in the cylinder in the post injectionquantity Qpost based on the formula (2).

[0045] Then, the quantity HCact of the hydrocarbon actually supplied tothe DPF 3 is calculated from the combustion-originated quantity HCcom ofthe hydrocarbon generated through the combustion in the engine 1 and thesupply-originated quantity HCsup originating from the post injection,based on the formula (1).

[0046] Then, in Step S107, based on the relationship shown in FIG. 2,the upper limit value HCup of the actually supplied quantity HCact forpreventing the hydrocarbon poisoning is calculated in accordance withthe present temperature Tin of the exhaust gas upstream of the DPF 3,which is sensed in Step S102. Then, in Step S108, the quantity HCact ofthe hydrocarbon actually supplied to the DPF 3 calculated in Step S106is compared with the upper limit value HCup calculated in Step S107.Thus, it is determined whether the quantity HCact of the hydrocarbonactually supplied to the DPF 3 is equal to or less than the upper limitvalue HCup. If the quantity HCact of the hydrocarbon actually suppliedto the DPF 3 is greater than the upper limit value HCup, the processingproceeds to Step S109 and the post injection quantity Qpost is decreasedto corrected post injection quantity Qpost' so that the quantity HCactof the hydrocarbon actually supplied to the DPF 3 coincides with theupper limit value HCup. Thus, the post injection quantity Qpost isrestricted so that the actually supplied quantity HCact does not exceedthe upper limit value HCup.

[0047] Then, the processing proceeds to Step S110 and the post injectionis performed based on the corrected post injection quantity Qpost'. Inthe case where the quantity HCact of the hydrocarbon actually suppliedto the DPF 3 is equal to or less than the upper limit value HCup in StepS108, the processing proceeds to Step S110 and the post injection isperformed.

[0048]FIG. 7 is a time chart showing an effect of the presentembodiment. Conventionally, the supply of the large amount of thehydrocarbon is started at timing t1 as shown by a broken line “a” inFIG. 7 in order to combust the particulate matters even if thetemperature Tin of the exhaust gas upstream of the DPF 3 is relativelylow. Therefore, the hydrocarbon poisoning can be caused easily and thetemperature TDPF of the DPF 3 does not increase quickly as shown by abroken line “c” in FIG. 7 because the catalytic reaction is hindered. Tothe contrary, in the present embodiment, the quantity HCact of thehydrocarbon supplied to the DPF 3 (the summation of thecombustion-originated quantity HCcom and the supply-originated quantityHCsup) is regulated as shown by a solid line “b” in FIG. 7 in order toprevent the hydrocarbon poisoning. Therefore, the temperature TDPF ofthe DPF 3 can be increased quickly as shown by a solid line “d”. As aresult, the particulate matters accumulated on the surface of the DPF 3can be combusted effectively, and the harmful components included in theexhaust gas can be purified effectively.

[0049] As explained above, in the present embodiment, the hydrocarbonpoisoning can be precluded and the deterioration of the performance ofthe catalyst due to the adhesion of the hydrocarbon can be avoided.Moreover, the deterioration of the fuel consumption due to the operationfor eliminating the hydrocarbon poisoning can be avoided.

[0050] The present invention exerts a great effect when the presentinvention is applied to the DPF having the oxidation catalyst, to whicha large amount of the hydrocarbon is supplied in order to combust theparticulate matters. The present invention can be applied to the exhaustgas after-treatment device supporting other catalysts. Morespecifically, the present invention can be applied to a catalyst systememploying a DPF with an oxidation catalyst, a nitrogen oxide removalcatalyst, an oxidation catalyst or a three-way catalyst, or acombination of them such as a nitrogen oxide removal catalyst thatreduces and purifies the nitrogen oxides by supplying the hydrocarbon,or a catalyst system for purifying the hydrocarbon, carbon monoxide, theparticulate matters or the nitrogen oxides by combining the nitrogenoxide removal catalyst, the oxidation catalyst and the three-waycatalyst.

[0051] The temperature Tin of the exhaust gas upstream of the DPF 3varies greatly in accordance with the operating condition. Therefore, inthe case where the upper limit value HCup of the quantity HCact of thehydrocarbon supplied to the DPF 3 is calculated from the temperature Tinof the exhaust gas upstream of the DPF 3, an averaged value of theplurality of sampled values of the temperature Tin may be employed forthe sake of stable sensing. In the above embodiment, the hydrocarbonquantity sensing means calculates the quantity HCact of the hydrocarbonactually supplied to the DPF 3 based on the operating conditions of theengine 1 and the like. Alternatively, the quantity HCact of the actuallysupplied hydrocarbon may be sensed with a hydrocarbon sensor disposed ashydrocarbon quantity sensing means. Alternatively, the hydrocarbonsupply quantity controlling means may calculate the post injectionquantity Qpost in advance so that the quantity HCact of the actuallysupplied hydrocarbon does not exceed the upper limit value HCup,considering the hydrocarbon poisoning.

[0052] The present invention should not be limited to the disclosedembodiment, but may be implemented in many other ways without departingfrom the spirit of the invention.

What is claimed is:
 1. An exhaust gas purification system of an internalcombustion engine, the exhaust gas purification system comprising: anexhaust gas after-treatment device, which is disposed in an exhaustpassage of the engine and supports a catalyst; temperature sensing meansfor estimating temperature of the exhaust gas after-treatment device;hydrocarbon supplying means for supplying hydrocarbon to the exhaust gasafter-treatment device; and hydrocarbon supply quantity controllingmeans for determining an upper limit value of the permissible quantityof the hydrocarbon supplied to the exhaust gas after-treatment device inaccordance with the temperature of the exhaust gas after-treatmentdevice estimated by the temperature sensing means, and for controllingthe hydrocarbon supplying means so that the quantity of the hydrocarbonsupplied to the exhaust gas after-treatment device becomes equal to orless than the upper limit value.
 2. The exhaust gas purification systemas in claim 1, wherein the hydrocarbon supplying means supplies thehydrocarbon to the exhaust gas after-treatment device by performing apost injection of fuel after a main injection of the fuel, by retardinginjection timing of the fuel, or by increasing a quantity of the exhaustgas recirculated into intake air.
 3. The exhaust gas purification systemas in claim 1, further comprising: hydrocarbon quantity sensing meansfor sensing the quantity of the hydrocarbon supplied to the exhaust gasafter-treatment device, wherein the hydrocarbon supply quantitycontrolling means controls the hydrocarbon supplying means so that thequantity of the hydrocarbon sensed by the hydrocarbon quantity sensingmeans becomes equal to or less than the upper limit value.
 4. Theexhaust gas purification system as in claim 3, wherein the hydrocarbonquantity sensing means calculates the quantity of the hydrocarbonsupplied to the exhaust gas after-treatment device by adding a quantityof unburned hydrocarbon generated through combustion in the engine tothe quantity of the hydrocarbon supplied by the hydrocarbon supplyingmeans.
 5. The exhaust gas purification system as in claim 1, wherein thetemperature sensing means senses temperature of the exhaust gas upstreamof the exhaust gas after-treatment device as the temperature of theexhaust gas after-treatment device.
 6. The exhaust gas purificationsystem as in claim 1, wherein the exhaust gas after-treatment deviceincludes at least one selected from the group of a diesel particulatefilter having an oxidation catalyst, a nitrogen oxide removal catalyst,an oxidation catalyst and a three-way catalyst.